Physicists Solve a Muon Mystery. Now, Old Results Don't Add Up

For over two decades, physicists have been perplexed by the muon's 'g-2' factor, an intrinsic magnetic property that seemed to deviate from theoretical predictions. This discrepancy suggested the exciting possibility of undiscovered particles influencing the muon's behavior, acting as a window into physics beyond the Standard Model.

• The muon, a heavier cousin of the electron, wobbles in a magnetic field, with its 'g-factor' (specifically, the 'g-2' excess wobble) being highly sensitive to interactions with all other particles, known and unknown.
• Early experimental results, like those from Brookhaven in 2001 and later Fermilab in 2021, showed a persistent deviation from theoretical predictions, fueling speculation about new physics.
• One primary theoretical approach, the 'data-driven method,' used experimental data from electron-positron collisions to infer the strong force's contribution to the muon's wobble. This method consistently supported the discrepancy.
• A different, purely theoretical approach called 'lattice QCD' (quantum chromodynamics), spearheaded by the BMW group, painstakingly simulated strong force interactions. After a decade of computational advancements, their 2021 results aligned perfectly with the experimental measurements, effectively resolving the original 'g-2' anomaly.
• This resolution, however, opened a new can of worms: the lattice QCD predictions directly contradict the experimental data inputs used by the data-driven method. Specifically, recent measurements of pion production rates at the VEPP-2000 collider in Siberia dramatically diverged from its own past results and those from other colliders like BABAR.
• The new VEPP-2000 data align with the lattice QCD predictions, while older data and other experiments support the data-driven method's inputs. This creates a fundamental conflict within experimental particle physics itself, questioning which experimental results for quark interactions are truly correct.

Physicists are now faced with the task of understanding these conflicting experimental results. The original muon mystery might be solved, but it has been replaced by a potentially more profound puzzle: an inconsistency in the experimental bedrock of particle physics, which could still hint at undiscovered phenomena or simply reveal overlooked experimental nuances.